The forming of articles comprised of a thermosetting polymer is known. It is also known that the formation of such articles oftentimes may be difficult, and that the techniques utilized may give rise to various drawbacks. In short, the known processes may not be wholly considered commercially feasible.
It is also known that a thermosetting resin in sheet form, containing filler and preimpregnated with a reinforcing glass fiber, may be subjected only to very limited pressure, without the sheet undergoing considerable creep, in making numerous plane or slightly curved panels. The procedure followed may be likened to that of shaping of sheet metal.
If articles of more complicated outline are to be formed, these articles of thermosetting resins may be injection molded. The molding process normally is carried out at high pressure levels as may be the case with thermoplastic resins. However, this process is expensive. The process, thus, is limited in use to that of molding complex pieces. Further, the process is not without various drawbacks introduced because of the resistance to molding imparted by the reinforcing fibers.
Another process of the prior art provides for the medium pressure molding of a material which may be in a pasty state. The molding operation is carried out within a cavity formed by a pair of plates of a standard press. While this process makes possible the formation of articles of variable thickness, and articles which exhibit considerable ribbings, such as curved panels or boxes of varying type, it has not been possible to utilize the process in the formation of complex shapes as may be formed in the implementation of a high pressure, injection molding process.
To reach industrial and commercially feasible rates in the forming of articles, it is normally required to use heat so that after a possible gelling stage there may be a rapid and complete polymerization of the resin. For example, in the process of molding a resin in the pasty state at a medium pressure value, the prior art has resorted to the use of plates of a standard press which are heated by conduction. The heat may derive from the circulation of a heat-carrying fluid.
While the heat conductive capacity of the plates may be satisfactory for purposes of supplying the level of heat for rapid and complete polymerization of the resin, the overall operation is made difficult because of the poor capacity of the materials to conduct heat. The difficulty becomes more pronounced as the thickness of the article which is to be formed increases, say, to several millimeters. Also, occasionally, the polymerization reaction is of an exothermic nature so that rather than taking in heat, the material gives up some supplemental heat.
In short, under normal operating conditions, it is difficult to obtain a homogeneous refluidizing and, consequently, a uniform creep and core polymerization of material to be formed utilizing an external heating agency.
Internal heating of various thermosetting resins, such as, in particular, phenolic resins of polyesters, under the effect of dielectric losses from a high frequency field and through use of the properties of those resins, is known, also. This practice makes it possible to achieve a more uniform, homogeneous heating. This assumes, however, that a sufficient uniformity can be obtained in the electric field. The heat conductivity of the plates of the press and heat loss in the walls may easily result in a drawback opposite to that previously discussed.
In view of this drawback, it has been a suggestion to heat the material before introduction into the mold. The preheating of the material, to a temperature below the gelling temperature, serves to increase the time given to maturation and reduces the temperature gradients without keeping the article being formed in the mold for an excessive period of time.
The preheating of the material provides several advantages. To this end, the preheating of the material facilitates the creep of the mass of material to be molded, and it provides for good filling of the mold. The preheating of material, also, serves to accelerate the rates by shortening the curing time of the material.
In the simplest of situations, the outside face of the mold walls may be provided with a heating system for external heating of the material. The source of external heat may be that of a heat-carrying fluid. These outside faces, also, may serve as electrodes making it possible, after the application of pressure to the material that is molded, to heat the material both internally (by a high frequency field) and externally (by conduction of heat to the object that is being formed).
It has also been proposed to neutralize the cold wall effect of the mold by use of a sheet which previously was polymerized. The polymerized sheet will serve as a facing, to insulate the core of the piece to be molded from the surfaces provided for final forming. On the other hand, it has been suggested to provide the walls of the mold along the mold cavity with a thin inside wall. The walls of the mold, then, could be thermally insulated from the pressure resistant bearing structure so that the walls, constituting the plates of a HF heating capacitor, exhibit only slight thermal inertia. In this manner, the material is not excessively cooled.
Finally, it has been proposed, and it has been found possible, to heat the molded piece while on the mold externally by conduction before the piece is transferred into an oven for polymerization by dielectric heating. Conversely, to assure polymerization, the material may be preheated under high frequency before molding, and then heated during molding by conduction of heat from the mold. Thus, with the latter, crosslinking is completed within the mold without otherwise compensating for possible temperature deviation observed from one stage to another.
One elaborate process is disclosed in French Pat. No. 1,232,996. The process follows several phases, corresponding to three stages of temperature rise. To this end, the process describes an optional preheating, preferably through external preheating, the molding of the material to a desired size and configuration, a rapid dielectric heating under high frequency to cause gelling, and, then, final heating by conduction to complete polymerization.